Trichomonas vaginalis (Tv) is the causative agent of trichomoniasis, the most prevalent, non-viral, sexually- transmitted infection, infecting ~ billion people worldwide annually. Tv infection is associated with adverse inflammatory sequelae, that can contribute to pregnancy complications, the spread of HIV and increased metastasis of urogenital cancers. The incidence of infection and an increase in the number of drug resistant Tv strains underscore the need to develop new chemotherapeutic and vaccine design strategies. To achieve these goals, a better understanding the immune response to Tv is imperative. Surprisingly little is known about immune responses to the parasite; however, persistence of trichomoniasis in untreated individuals and frequent partner re-infection suggest that immunity to Tv is often ineffective. Neutrophils (PMN) are abundant at the site of Tv infection. We recently reported that PMN kill Tv using a novel mechanism called trogocytosis (trogo=to nibble), by which PMN take bites from Tv, leading to parasite death. Tv strains killed by trogocytosis (referred to as trogocytosis sensitive: TvTS) are killed rapidly (5 mins ? 2 hrs) in in vitro assays using human PMN. We have also identified strains that are trogocytosis resistant (TvTR), which are killed upon prolonged (~16 hrs) incubation with PMN, by an undefined mechanism. Many Tv clinical isolates naturally harbor a bacterial symbiont, Mycoplasma hominis (Mh). Our proposed studies focus on TvTS (early-stage killing) and TvTR (late-stage killing) strains, in the presence or absence of Mh. Preliminary data suggest that host clearance of Tv is subverted by TvTR strains, at least in part due to the secretion of a Mh nuclease that degrades DNA NETs cast by PMN in late-stage killing. Our specific aims are: Aim1: To determine the mechanism(s) used by neutrophils (PMN) to kill a trogocytosis-resistant Tv strain (TvTR). PMN killing of TvTR using NETosis, extracellular degranulation or engulfment will be determined, to test the hypothesis that PMN deploy NETosis to control trogocytosis-resistant strains of Tv. Aim 2: To determine whether: (a) Mh0730 degrades NETs, (b) Mh0730 rescues TvTR from PMN killing [and (c) loss of Mh0730, using gene knock out technology, makes TvTR more susceptible to PMN killing.] These studies test whether the symbiont nuclease rescues Tv from NETosis. The proposed research will increase our understanding of the immune response to Tv and the role of a common symbiont in modulating responses. [Knowledge gained will be instrumental in understanding how this prevalent, neglected pathogen avoids natural clearance by human immune cells and will be useful to inform design of therapies to target the parasite and its associated reproductive pathology.]